The present invention relates to a control apparatus for an engine, particularly to a control apparatus for an engine which performs an engine stop control.
Conventionally, it is known that when an engine installed in a vehicle gradually decelerates to stop, the engine and the mount thereof resonate and the vehicle may vibrate. Specifically, a frequency range where such resonance occurs (hereinafter, referred to as the “resonant frequency range”) exists within a low engine speed range, and the vibration may occur when the engine speed passes through the resonant frequency range while the engine decelerates.
Therefore, techniques for suppressing such vibration caused in stopping the engine are proposed. JP2000-240483A discloses such a technique. According to JP2000-240483A, when an engine stop condition is satisfied, the engine speed is increased, a throttle valve is operated in a closing direction, and then the engine is stopped. In this manner, an intake air amount per engine cycle is reduced to suppress a change in engine load during a compression stroke and an expansion stroke. Thus, engine speed variation is suppressed, and the vibration caused by stopping the engine is suppressed.
According to JP2000-240483A, the engine speed is increased and then the throttle valve is operated in the closing direction to generate negative pressure within an intake passage. However, this method takes time in generating sufficient negative pressure within the intake passage (i.e., the pressure within the intake passage cannot be reduced quickly). The amplitude of the vibration caused within the resonant frequency range correlates with the negative pressure within the intake passage, specifically, the amplitude of the vibration becomes large if the negative pressure becomes low, and therefore, with the technique described in JP2000-240483A, sufficient negative pressure cannot be generated while the engine speed passes through the resonant frequency range, and the amplitude of the vibration cannot suitably be reduced. Further, with the technique described in JP2000-240483A, since it takes time to generate sufficient negative pressure, it also takes time to drop the engine speed to zero. Therefore, a length of time that the engine speed remains within the resonant frequency range while it decelerates becomes long, and the vibration cannot suitably be suppressed.